Method and Chemistry for Selenium Electrodeposition

ABSTRACT

Techniques for electrodepositing selenium (Se)-containing films are provided. In one aspect, a method of preparing a Se electroplating solution is provided. The method includes the following steps. The solution is formed from a mixture of selenium oxide; an acid selected from the group consisting of alkane sulfonic acid, alkene sulfonic acid, aryl sulfonic acid, heterocyclic sulfonic acid, aromatic sulfonic acid and perchloric acid; and a solvent. A pH of the solution is then adjusted to from about 2.0 to about 3.0. The pH of the solution can be adjusted to from about 2.0 to about 3.0 by adding a base (e.g., sodium hydroxide) to the solution. A Se electroplating solution, an electroplating method and a method for fabricating a photovoltaic device are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/459,156 filed on Aug. 13, 2014 which is a divisional of U.S.application Ser. No. 12/878,811 filed on Sep. 9, 2010, now U.S. Pat. No.8,840,770, the contents of each of which are incorporated by referenceherein.

FIELD OF THE INVENTION

The present invention relates to electrodeposition and moreparticularly, to techniques for electrodepositing selenium(Se)-containing films.

BACKGROUND OF THE INVENTION

Selenium (Se) has been used for the production of solid-statephotoelements and current rectifiers. Se, however, has potentialapplication in semiconductor liquid junction solar cells and before theintroduction of digital photography it was used in the reproduction ofphotographs. In the solid state, Se has several allotropic forms.Crystalline allotropes of Se include two monoclinic forms and onehexagonal form. The amorphous and monoclinic forms of Se are dark red toblack and red, respectively. The hexagonal form is a semiconductor andit is gray. See, for example, A. K. Graham et al. “Electrodeposition ofAmorphous Selenium,” JECS, vol. 106, issue 8, pgs. 651-654 (1959)(hereinafter “Graham”). At room temperature, Se is a p-typesemiconductor with a bandgap energy of Eg=1.9 eV exhibiting a directoptical transition with a correspondingly high absorption coefficient.

Recently, there has been a demand for copper indium selenide (CIS),copper indium gallium selenide (CIGS) and copper zinc tin sulfide (CZTS)materials, all of which contain Se. These materials are being used asabsorber layers in photovoltaic devices. To form the absorber layer,evaporation methods are typically employed to deposit the variouscomponents of the material. Evaporation methods, however, are expensive,deposition rate is very low and a lot of material is lost during theprocess.

It is known that plating of Se-containing thin films by electrochemicalmethods (electrodeposition) is extremely difficult due the non-adherent,powdery and resistive nature of Se. There is very little literature onthe electroplating of Se films. In a few studies researchers were ableto plate, at best, a few monolayers of amorphous and resistive Se. See,for example, Graham (wherein amorphous Se was electroplated on nickel(Ni) up to a thickness of 30 micrometers (μm)). There is a large body ofliterature on Se being plated as an alloy with copper, indium andgallium (CIS/CIGS-alloy), but these studies do not apply toelectroplating only Se.

Therefore, improved techniques for electrodepositing Se-containing filmswould be desirable.

SUMMARY OF THE INVENTION

The present invention provides techniques for electrodepositing selenium(Se)-containing films. In one aspect of the invention, a method ofpreparing a Se electroplating solution is provided. The method includesthe following steps. The solution is formed from a mixture of seleniumoxide; an acid selected from the group consisting of alkane sulfonicacid, alkene sulfonic acid, aryl sulfonic acid, heterocyclic sulfonicacid, aromatic sulfonic acid and perchloric acid; and a solvent. A pH ofthe solution is then adjusted to from about 2.0 to about 3.0. The pH ofthe solution can be adjusted to from about 2.0 to about 3.0 by adding abase (e.g., sodium hydroxide (NaOH)) to the solution.

In another aspect of the invention, a Se electroplating solution isprovided. The solution includes selenium oxide; an acid selected fromthe group consisting of alkane sulfonic acid, alkene sulfonic acid, arylsulfonic acid, heterocyclic sulfonic acid, aromatic sulfonic acid andperchloric acid; a solvent; and a base, wherein a pH of the solution isfrom about 2.0 to about 3.0.

In yet another aspect of the invention, an electroplating method isprovided. The method includes the following steps. A Se electroplatingsolution is prepared by the steps of: forming the solution from amixture comprising selenium oxide, an acid selected from the groupconsisting of alkane sulfonic acid, alkene sulfonic acid, aryl sulfonicacid, heterocyclic sulfonic acid, aromatic sulfonic acid and perchloricacid and a solvent; and adjusting a pH of the solution to from about 2.0to about 3.0. A substrate is provided. A Se-containing film iselectroplated on the substrate using the solution as a plating bath.

In still yet another aspect of the invention, a method for fabricating aphotovoltaic device is provided. The method includes the followingsteps. A Se electroplating solution is prepared. The solution includesselenium oxide; an acid selected from the group consisting of alkanesulfonic acid, alkene sulfonic acid, aryl sulfonic acid, heterocyclicsulfonic acid, aromatic sulfonic acid and perchloric acid; a solvent; atleast one metal salt; and a base, wherein a pH of the solution is fromabout 2.0 to about 3.0. A substrate is provided. A Se-containingabsorber layer is electroplated on the substrate using the solution as aplating bath. A buffer layer is formed on the absorber layer. Atransparent electrode is formed on the buffer layer.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary methodology for preparinga selenium (Se) electroplating solution according to an embodiment ofthe present invention;

FIG. 2 is a diagram illustrating an exemplary methodology for forming aSe-containing film according to an embodiment of the present invention;

FIG. 3 is a diagram illustrating an exemplary photovoltaic device havingan absorber layer that may be fabricated using the methodology of FIG. 2according to an embodiment of the present invention;

FIG. 4 is a diagram illustrating an exemplary methodology forfabricating a photovoltaic device according to an embodiment of thepresent invention;

FIG. 5 is a graph illustrating plating properties of an exemplary Seonly film produced using the present techniques according to anembodiment of the present invention;

FIG. 6A is an x-ray diffraction (xrd) spectrum for an exemplary Se onlyfilm, produced using the present techniques, after an anneal for 7 hoursaccording to an embodiment of the present invention;

FIG. 6B is an xrd spectrum for an exemplary Se only film, produced usingthe present techniques, after an anneal for 1 hour according to anembodiment of the present invention;

FIG. 6C is an xrd spectrum for an exemplary Se only film, produced usingthe present techniques, as-plated (without anneal) according to anembodiment of the present invention;

FIG. 6D is an xrd spectrum for a substrate onto which the films of FIGS.6A-C are plated according to an embodiment of the present invention;

FIG. 7A is an image taken of the as-plated Se film of FIG. 6C accordingto an embodiment of the present invention;

FIG. 7B is an image taken of the Se film of FIG. 6B annealed for 1 houraccording to an embodiment of the present invention;

FIG. 8 is an x-ray fluorescence (xrf) spectrum for an exemplary Se onlyfilm produced using the present techniques according to an embodiment ofthe present invention;

FIG. 9 is a graph illustrating plating properties of an exemplarycopper-selenium (CuSe) alloy-containing film produced using the presenttechniques according to an embodiment of the present invention;

FIG. 10A is an xrd spectrum for an exemplary CuSe alloy-containing film,produced using the present techniques, after an anneal for 7 hoursaccording to an embodiment of the present invention;

FIG. 10B is an xrd spectrum for an exemplary CuSe alloy-containing film,produced using the present techniques, after an anneal for 1 houraccording to an embodiment of the present invention;

FIG. 10C is an xrd spectrum for an exemplary CuSe alloy-containing film,produced using the present techniques, as-plated (without anneal)according to an embodiment of the present invention;

FIG. 10D is an xrd spectrum for a substrate onto which the films ofFIGS. 10A-C are plated according to an embodiment of the presentinvention;

FIG. 11A is an image taken of the as-plated CuSe film from FIG. 10Caccording to an embodiment of the present invention;

FIG. 11B is an image taken of the as-plated CuSe film from FIG. 10Bannealed for 1 hour according to an embodiment of the present invention;

FIG. 12 is an xrf spectrum for an exemplary CuSe alloy-containing filmproduced using the present techniques according to an embodiment of thepresent invention;

FIG. 13A is a graph illustrating plating properties of an exemplary CuSefilm produced using the present techniques, wherein the acid in theplating solution is perchloric acid (HClO₄) according to an embodimentof the present invention;

FIG. 13B is a graph illustrating plating properties of an exemplary Sefilm produced using the present techniques, wherein the acid in theplating solution is HClO₄ according to an embodiment of the presentinvention;

FIG. 13C is a graph illustrating plating properties of an exemplary Sefilm produced using the present techniques, wherein the acid in theplating solution is HClO₄ and wherein a concentration of the Se used inthe solution is varied according to an embodiment of the presentinvention;

FIG. 13D is a graph illustrating plating properties of an exemplary Sefilm produced using the present techniques, wherein the acid in theplating solution is HClO₄ and wherein a concentration of both the Se andthe acid used in the solution are varied according to an embodiment ofthe present invention;

FIG. 14A is an xrd spectrum for an exemplary CuSe alloy-containing film,produced using the present techniques, wherein the electroplating bathcontains HClO₄, after an anneal for 7 hours according to an embodimentof the present invention;

FIG. 14B is an xrd spectrum for an exemplary CuSe alloy-containing film,produced using the present techniques, wherein the electroplating bathcontains HClO₄, after an anneal for 1 hour according to an embodiment ofthe present invention;

FIG. 14C is an xrd spectrum for an exemplary CuSe alloy-containing film,produced using the present techniques, wherein the electroplating bathcontains HClO₄, as-plated (without anneal) according to an embodiment ofthe present invention;

FIG. 14D is an xrd spectrum for a substrate onto which the films ofFIGS. 14A-C are plated according to an embodiment of the presentinvention;

FIG. 15A is an image taken of the as-plated CuSe film from FIG. 14Caccording to an embodiment of the present invention;

FIG. 15B is an image taken of the CuSe film from FIG. 14B annealed for 1hour according to an embodiment of the present invention; and

FIG. 16 is an xrf spectrum for an exemplary CuSe alloy-containing filmproduced using the present techniques, wherein the electroplating bathcontains HClO₄, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Provided herein are techniques that remedy the above-described problemsassociated with electrodepositing selenium (Se) films. The terms“electrodeposition” and “electroplating” are being used interchangeablyherein. FIG. 1, for example, is a diagram illustrating an exemplarymethodology 100 for preparing a Se electroplating solution. In step 102,the solution is formed from a mixture of selenium oxide, a sulfonic acidor perchloric acid (HClO₄) and a solvent.

Any form of selenium oxide may be used. According to an exemplaryembodiment, the selenium oxide is selenium dioxide (SeO₂). Suitablesulfonic acids include, but are not limited to, alkane sulfonic acid(e.g., methane sulfonic acid (MSA)), alkene sulfonic acid, aryl sulfonicacid, heterocyclic sulfonic acid and aromatic sulfonic acid.

While either a sulfonic acid or HClO₄ may be used in the electroplatingsolution, the sulfonic acid tends to perform better, see below. With allother acids tested, only a powdery deposit is obtained during plating.The powdery deposit is non-adherent to its substrate and easily washesaway during rinsing. According to an exemplary embodiment, the solutioncontains from about 0.01 molar (M) to about 1M Se in from about 0.05M toabout 1M acid (sulfonic acid or HClO₄), e.g., from about 0.1M to about0.5M Se in from about 0.1M to about 0.6M acid (sulfonic acid or HClO₄).Suitable solvents include, but are not limited to, one or more of water,glycerol and an ionic liquid. Advantageously, the electroplatingsolution does not contain any complexing agents. Complexing agentsintroduce impurities into the deposited film. Furthermore, complexingagents oxidize and are difficult to maintain in the plating bath.

At this point, the solution will have a pH of about 0. In step 104, thepH of the solution is adjusted with a base, such as sodium hydroxide(NaOH), to from about 2.0 to about 3.0.

Optionally, in step 106, one or more additives may be added to thesolution. These additives can include one or more organic additives, oneor more metalloid halides and/or one or more sources of ions. Suitableorganic additives include those having at least one nitrogen atom and atleast one sulfur atom, such as thiourea or thiazine, or benzenesulfonicacid (BSA). The organic additive serves to aid in grain refinement, andto provide more smooth and uniform plating. According to an exemplaryembodiment, the organic additive is present in the solution at aconcentration of from about 1 parts per million (ppm) to about 10,000ppm.

Suitable metalloid halides include, but are not limited to, bismuthchloride (BiCl₃). Suitable sources of ions include, but are not limitedto, potassium chloride (KCl) and/or sodium chloride (NaCl). Themetalloid halide and/or ion source additives advantageously help toproduce a brighter deposit and help to refine the grains/morphology ofthe film.

Use of the solution as described above will result in the plating of afilm containing Se only. However, the present techniques are alsoapplicable to the plating of Se alloy films. By way of example only, instep 108, one or more metal salts such as a copper (Cu) salt (e.g.,copper sulfate (CuSO₄), copper chlorite (Cu(ClO₂)₂) and/or coppernitrate (Cu(NO₃)₂), a zinc (Zn) salt (e.g., zinc sulfate (ZnSO₄), zincchlorite (ZnClO₄) and/or zinc nitrate (Zn(NO₃)₂), an indium (In) salt(e.g., indium sulfate (InSO₄), indium chlorite (In(ClO₂)₃ and/or indiumnitrate (In(NO₃)₃), a gallium (Ga) salt (e.g., gallium sulfate (GaSO₄),gallium chlorite (GaClO₄) and/or gallium nitrate (GaN₃O₉) and/or a tin(Sn) salt (e.g., tin sulfate (SnSO₄), tin chlorite (SnClO₃)₂ and/or tinnitrate (Sn(NO₃)₂) are optionally added to the solution in order toproduce a corresponding Se alloy-containing film, e.g., copper-selenium(CuSe) and zinc-selenium (ZnSe). ZnSe, for example, is a good n-typebuffer material that may be used in the fabrication of a photovoltaicdevice, as described below.

The metal salts can be used in combination. By way of example only, toproduce a Se/Sn/Cu/Zn-containing film, such as Cu₂ZnSn(S/Se)₄ (CZTS), acombination of any one of the above-listed Sn salts, Cu salts and Znsalts can be added to the solution, each at a concentration of fromabout 0.1M to about 0.5M, in order to plate CZTS. To produce a Cu/In/Se-or Cu/In/Ga/Se-containing film, such as copper indium selenide (CIS) orcopper indium gallium selenide (CIGS), respectively, a combination ofany one of the above-listed Cu salts and In salts (and Ga salts forCIGS) can be added to the solution, each at a concentration of fromabout 0.1M to about 0.5M, in order to plate CIS or CIGS.

According to an exemplary embodiment, one liter (L) of the solution isprepared by placing from about 200 milliliters (ml) to about 400 ml ofde-ionized water in a flask. SeO₂ at a concentration of from about 0.1Mto about 1.0M is then added to the flask. 0.5M MSA or HClO₄ is thenadded to the flask. After stirring the SeO₂ dissolves and the solutionbecomes colorless.

If electrodeposition of an elemental Se layer is desired, thenadditional de-ionized water is added to the flask to bring the totalvolume to 1 L (pH of from about 0.1 to about 0.5). The solution is thenready for electroplating. On the other hand, if electrodeposition of Sein combination with a metal, such as copper selenium (CuSe) or zincselenium (ZnSe) is desired, then the corresponding metal salt is firstadded to the solution. For example, to prepare a CuSe electroplatingsolution, Cu salt is added to the flask (millimole (mM) to Mconcentration). For plating a Se rich CuSe film, a mM amount of Cu saltis required in the solution. For plating a Se poor CuSe film, a M amountof Cu salt is required in the solution. Additional de-ionized water isthen added to the flask to bring the total volume to 1 L.

To prepare a ZnSe electroplating solution, for example, Zn salt is addedto the flask (mM to M concentration). For plating a Se rich ZnSe film, amM amount of Zn salt is required in the solution. For plating a Se poorZnSe film, a M amount of Zn salt is required in the solution. Additionalde-ionized water is then added to the flask to bring the total volume to1 L.

The solution may then be used as a plating bath to electroplate aSe-containing film on a substrate. FIG. 2 is a diagram illustratingexemplary methodology 200 for forming a Se-containing film. The film mayinclude Se alone, or in combination with one or more metals, such as Cu,Zn, In, Ga and Sn. Advantageously, the present techniques can beemployed to fabricate copper indium selenide (CIS), copper indiumgallium selenide (CIGS) and copper zinc tin sulfide/selenide (CZTS)materials for use, e.g., in photovoltaic devices. See below.

In step 202, a Se electroplating solution is prepared. The process forpreparing the Se electroplating solution was described in conjunctionwith the description of FIG. 1, above, and that description isincorporated by reference herein.

In step 204, a substrate is provided. The substrate can be any substrateon which the deposition of a Se-containing film is desired. According toan exemplary embodiment, the present techniques are employed in theformation of a Se-containing absorber layer for a photovoltaic device.In that instance (as will be described in detail below), the substratecan be the photovoltaic device substrate (e.g., a molybdenum (Mo)-coatedglass substrate).

In step 206, electroplating is then used to form a Se-containing film onthe substrate using the solution as the plating bath. During theelectroplating process, the substrate is placed directly in the bath.Electroplating techniques, parameters and an exemplary electroplatingcell which are suitable for use with the present electroplatingsolutions are described, for example, in U.S. patent application Ser.No. 12/878,746, entitled “Structure and Method of Fabricating a CZTSPhotovoltaic Device by Electrodeposition” (hereinafter “U.S. patentapplication Ser. No. 12/878,746”), now U.S. Pat. No. 8,426,241, thecontents of which are incorporated by reference herein. Of course, theexact composition of the film produced will depend on the composition ofthe electroplating solution. As described above, the electroplatingsolution may contain Se, either alone, or in combination with one ormore metal salts. For instance, when the electroplating solutionincludes both Cu and Zn metal salts, then the film produced will containan alloy of Se, Cu and Zn. As described below, the resultant films areSe rich.

According to an exemplary embodiment, the electroplating is carried outat room temperature, i.e., from about 18 degrees Celsius (° C.) to about24° C. The duration of the plating can be tailored to the desiredthickness of the film (see below).

Optionally, in step 208, the film is then annealed. This anneal isoptional based on the intended use of the film. By way of example only,when the film is being used in the production of a photovoltaic deviceabsorber layer conductivity is important. As-plated Se is amorphous andexhibits poor conductivity, which can be remedied by the anneal.Accordingly, in this instance the annealing step would be favorable toconvert the film into a crystalline, conductive layer. According to anexemplary embodiment, the film is annealed at a temperature of fromabout 100° C. to about 300° C., e.g., about 150° C., for a duration offrom about 30 minutes to about 60 minutes.

One notable advantage of the present techniques is that theelectroplating process and Se electroplating solutions described hereincan be used to form Se alloy films (e.g., CIS, CIGS, CZTS) that canserve as the absorber layer in photovoltaic devices. An exemplaryphotovoltaic device and method for the fabrication thereof will now bedescribed. It is to be understood however that there are many differentpossible photovoltaic device configurations, and the particularconfiguration described below is provided merely to illustrate thepresent techniques.

FIG. 3 is a diagram illustrating exemplary photovoltaic device 300.Photovoltaic device 300 includes a substrate 302, p-type absorber layer304 adjacent to substrate 302, n-type buffer layer 306 adjacent to aside of absorber layer 304 opposite substrate 302 and transparentelectrode 308 adjacent to a side of buffer layer 306 opposite absorberlayer 304. According to an exemplary embodiment, substrate 302 includesa glass, metal or plastic substrate 302 a that is coated with a material302 b such as Mo. A metal layer 303 (e.g., a Cu layer) may be present inbetween substrate 302 and absorber layer 304. A conductor layer isneeded in order to electroplate the absorber layer on the substrate(see, for example, FIG. 4, described below). In this example, metallayer 303 serves this purpose. Further, metal layer 303 prevents theundesirable oxidation of the Mo. Metal layer 303 can serve as a bottomelectrode for the device.

In this example, absorber layer 304 is an Se-containing p-type absorbermaterial (e.g., CIS, CIGS or CZTS) formed using the present techniques,e.g., wherein the plating bath contains Se in combination with theappropriate metal salts, as described above. The process for forming theabsorber layer is described further below.

Buffer layer 306 includes an n-type material such as cadmium sulfide(CdS) or ZnSe. Like with the absorber layer materials, ZnSe can beplated using the present Se electroplating solution with the addition ofa Zn metal salt. See below. Transparent electrode 308 can includematerials, such as aluminum (Al) or n-doped zinc oxide (ZnO) orindium-tin-oxide (ITO) and intrinsic ZnO.

FIG. 4 is a diagram illustrating exemplary methodology 400 forfabricating a photovoltaic device, such as photovoltaic device 300 (ofFIG. 3). In step 402, a substrate is provided. According to an exemplaryembodiment, the substrate is a glass substrate coated with Mo. In step404, a metal layer is formed on the substrate, i.e., on the Mo-coatedsurface of the glass substrate. As described above, the metal layerserves as a conductive layer for electroplating and prevents oxidationof the Mo. The metal layer will serve as a bottom electrode of thedevice. According to an exemplary embodiment, the metal layer includesCu and is formed using sputtering to a thickness of from about 10nanometers (nm) to about 100 nm.

In step 406, a p-type absorber layer is formed adjacent to thesubstrate. Namely, the substrate (with the metal layer) is placeddirectly into a plating bath solution prepared as described, forexample, in conjunction with the description of FIG. 1, above. In thiscase, the electroplating solution, in addition to Se, contains one ormore metal salts. The particular metal salt(s) used depends on thedesired composition of the absorber layer. By way of example only, theelectroplating solution would contain a Cu salt and an In salt if a CISabsorber layer is desired. The further addition of a Ga salt wouldresult in a CIGS absorber layer. A Cu salt, a Zn salt and a Sn saltwould be employed for a CZTS absorber layer. Suitable Cu, In, Ga, Zn andSn salts were provided above. Electroplating is then used to form theabsorber layer (e.g., CIS, CIGS, CZTS) on the substrate over the metallayer. The plating time can be tailored to the desired thickness of theresultant absorber layer, with a longer plating time being used toattain a thicker layer. According to an exemplary embodiment, theabsorber layer is formed on the substrate to a thickness of from about 1nm to about 1 micrometer (μm).

The absorber layer produced in this manner will be an alloy of Se andthe appropriate metal(s). It is also possible to plate a plurality oflayers, each layer containing one or more components of the absorberlayer, and then anneal the layers to intersperse the components and formthe absorber layer. This technique is described in U.S. patentapplication Ser. No. 12/878,746. To do so may require plating layerssuch as Cu-containing and Zn-containing layers. Techniques for preparinga Zn plating solution for electrodepositing a Zn-containing metal layer,for example, are described in U.S. patent application Ser. No.12/878,787, entitled “Zinc Thin Films Plating Chemistry and Methods,”the contents of which are incorporated by reference herein.

As highlighted above, it is advantageous to employ an anneal, forexample, at a temperature of from about 100° C. to about 300° C., e.g.,about 150° C., for a duration of from about 30 minutes to about 60minutes, to convert the electrodeposited Se film into a crystalline,conductive layer. This anneal is performed in step 407.

The present method, as compared to conventional evaporation orsputtering, will be cheapest, allow a high rate of deposition as well asimprove the adhesion of the absorber layer to its back contactmaterials. When a CIS, CIGS or CZTS material is fabricated by thedeposition of a stack of metal thin films such as a stack of Cu, In andGa metal films or a stack of Cu, Zn and Sn metal films there is a highvolume expansion during subsequent selenization (the process by which Seis introduced into the stack), which can lead to poor adhesion (andpossibly delamination) of the absorber layer from the back contactmaterial. Advantageously, the present techniques permit plating of Sethus avoiding the need for selenization, and the problems associatedtherewith. Advantageously, the present technique permits the Se to bedeposited as an interlayer in the stack (as opposed to being introducedduring selenization) which altogether avoids the problem of large volumeexpansion and loss of adhesion of the chalcopyrite material. Evaporationand/or sputtering, aside from the high cost, also have poor selectivity.Electrodeposition has 100% selectivity and is a low cost method ofdeposition utilizing solutions.

In step 408, an n-type buffer layer is formed adjacent to a side of theabsorber layer opposite the substrate. According to an exemplaryembodiment, the buffer layer includes CdS and is deposited onto theabsorber layer using chemical bath deposition or a spray technique.Alternatively, the buffer layer can include ZnSe and can beelectrodeposited using the present Se electroplating solution as aplating bath. The electroplating solution in this case would contain Seand one of the aforementioned Zn salts. Thus, the substrate can beplaced in a first one of the present Se electroplating solutions(containing Se in combination with the appropriate metal salt(s) for theabsorber layer) to plate the absorber layer (step 406). Once theabsorber layer is plated, the structure can be removed from the firstbath, annealed (step 407), and then placed in a second one of thepresent Se electroplating solutions (containing Se in combination with aZn salt) to plate the buffer layer. With whichever buffer layermaterial/deposition technique is employed, the absorber layer is formedhaving a thickness of, e.g., from about 20 nm to about 40 nm. Since thebuffer layer contains an n-type material, the buffer layer will form ap-n junction with the p-type absorber layer.

In step 410, a transparent electrode is formed on the buffer layer(adjacent to a side of the buffer layer opposite the absorber layer).According to an exemplary embodiment, the transparent electrode includesAl or n-doped ZnO or a combination of ITO and intrinsic ZnO and isdeposited onto the buffer layer using a sputtering technique to athickness of from about 250 nm to about 350 nm.

The present techniques are further described by way of reference to thefollowing non-limiting examples. FIG. 5 is a graph 500 illustratingplating properties of an exemplary Se only film produced using thepresent techniques. The electroplating solution in this examplecontained 0.2M Se in 0.5M MSA. A standard pulse-reverse electroplatingprocess was employed. In graph 500, electrode voltage (E) (measured involts (V)) vs. saturated calomel reference electrode (SCE) is plotted onthe x-axis and current density (measured in milliamps per squarecentimeter (mAcm⁻²) is plotted on the y-axis. The current density in thesecond cycle is low due to the resistive nature of the deposited Sefilms. The surface of electroplated Se thin films becomes oxidizedduring the reverse cycle when the electrode is subjected to the morepositive voltage and thus the film became resistive and hence during thesecond cycle low current density was observed.

FIGS. 6A-D are x-ray diffraction (xrd) spectra 600A-D for an exemplarySe only film produced using the present techniques: after an anneal ofthe film for 7 hours at 100° C., after an anneal of the film for 1 hourat 100° C., the film as-plated and for a substrate (onto which the filmwas plated), respectively. In each spectra, beam angle (2θ) is plottedon the x-axis and intensity (measured in atomic units (a.u.)) is plottedon the y-axis. The electroplating solution in this example contained0.2M Se in 0.5M MSA. Current density (J)=10 mAcm⁻² and plating time(t)=60 seconds. No noticeable change was observed in the xrd spectrumafter the 7 hour annealing. The substrate in this example was aMo-coated glass substrate with a metal (e.g., Cu) layer on top (seedescription above).

FIGS. 7A and 7B are images 700A and 700B taken of the Se film from FIGS.6A-C (see above) as-plated and after the anneal at 100° C. for 1 hour,respectively. By comparison with image 700A, image 700B shows largegrain growth after annealing.

FIG. 8 is an x-ray fluorescence (xrf) spectrum 800 for an exemplary Seonly film produced using the present techniques. In the spectrum, energy(measured in kiloelectron volts (keV)) is plotted on the x-axis andcounts are plotted on the y-axis. The electroplating solution in thisexample contained 0.2M Se in 0.5M MSA. Current density (J)=10 mAcm⁻² andplating time (t)=60 seconds. The substrate in this example was aMo-coated glass substrate with a metal (e.g., Cu) layer on top (seedescription above). There was no Cu in the plating solution. Thereforethe low intensity Cu peak is due to the substrate.

Next, Se alloy films were produced. FIG. 9 is a graph 900 illustratingplating properties of an exemplary CuSe alloy-containing film producedusing the present techniques. The electroplating solution in thisexample contained 0.2M Se in 0.5M MSA with 0.025 millimolar (mM) Cu. Astandard pulse-reverse electroplating process was employed. In graph900, electrode voltage (E) (measured in V) vs. SCE is plotted on thex-axis and current density (measured in mAcm⁻²) is plotted on they-axis.

FIGS. 10A-D are xrd spectra 1000A-D for an exemplary CuSealloy-containing film produced using the present techniques: after ananneal of the film for 7 hours at 100° C., after an anneal of the filmfor 1 hour at 100° C., the film as-plated and for a substrate (ontowhich the film was plated), respectively. In each spectra, beam angle(2θ) is plotted on the x-axis and intensity (measured in a.u.) isplotted on the y-axis. The electroplating solution in this examplecontained 0.2M Se in 0.5M MSA with 0.025 mM Cu. No noticeable change wasobserved in the xrd spectrum after the 7 hour annealing. The substratein this example was a Mo-coated glass substrate with a metal (e.g., Cu)layer on top (see description above).

FIGS. 11A and 11B are images 1100A and 1100B taken of the Se/Cu filmfrom FIGS. 10A-C (see above) as-plated and after the anneal at 100° C.for 1 hour, respectively. By comparison with image 1100A, image 1100Bshows large grain growth after annealing.

FIG. 12 is an xrf spectrum 1200 for an exemplary CuSe alloy-containingfilm produced using the present techniques. In the spectrum, energy(measured in keV) is plotted on the x-axis and counts are plotted on they-axis. The electroplating solution in this example contained 0.2M Se in0.5M MSA and 0.025 mM Cu. Current density (J)=10 mAcm⁻² and plating time(t)=60 seconds. The solution contained Cu²⁺ in a low concentration andthe alloy was still Se rich.

Next, HClO₄ (instead of MSA) was used in the plating solution. FIGS.13A-D are graphs 1300A-D, respectively, illustrating plating propertiesof exemplary films produced using the present techniques, wherein theacid in the plating solution is HClO₄. Specifically, in a first sample,the electroplating solution contained 0.2M Se in 0.5M HClO₄ with 0.025mM Cu (see graph 1300A). In the second sample, the electroplatingsolution contained 0.2M Se in 0.5M HClO₄ (see graph 1300B). In the thirdsample, the electroplating solution contained 10 mM Se in 0.5M HClO₄(see graph 1300C). In the fourth sample, the electroplating solutioncontained 10 mM Se in 0.1M HClO₄ (see graph 1300D). A standardpulse-reverse electroplating process was employed. In each graph1300A-D, electrode voltage (E) (measured in V) vs. SCE is plotted on thex-axis and current density (measured in mAcm⁻²) is plotted on they-axis. It is notable that the y-scale for the graphs are not scaledequally.

FIGS. 14A-D are x-ray diffraction (xrd) spectra 1400A-D for an exemplaryCuSe alloy-containing film produced using the present techniques,wherein the electroplating bath contains HClO₄: after an anneal of thefilm for 7 hours at 100° C., after an anneal of the film for 1 hour at100° C., the film as-plated and for a substrate (onto which the film wasplated), respectively. In each spectrum, beam angle (2θ) is plotted onthe x-axis and intensity (measured in a.u.) is plotted on the y-axis.The electroplating solution in this example contained 0.2M Se in 0.5MHClO₄ with 0.025 mM Cu. The substrate in this example was a Mo-coatedglass substrate with a metal (e.g., Cu) layer on top (see descriptionabove).

FIGS. 15A and 15B are images 1500A and 1500B taken of the Se/Cu filmfrom FIGS. 14A-C (see above) as-plated and after the anneal at 100° C.for 1 hour, respectively. By comparison with image 1500A, image 1500Bshows large grain growth after annealing.

FIG. 16 is an xrf spectrum 1600 for an exemplary CuSe alloy-containingfilm produced using the present techniques, wherein the electroplatingbath contains HClO₄. In the spectrum, energy (measured in keV) isplotted on the x-axis and counts are plotted on the y-axis. Theelectroplating solution in this example contained 0.2M Se in 0.5M HClO₄and 0.025 mM Cu. Current density (J)=10 mAcm⁻² and plating time (t)=60seconds. The solution contained Cu²⁺ in a low concentration and thealloy was still Se rich.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. An electroplating method, comprising the stepsof: preparing a selenium electroplating solution by the steps of:forming the solution from a mixture comprising selenium oxide, an acidselected from the group consisting of alkane sulfonic acid, alkenesulfonic acid, aryl sulfonic acid, heterocyclic sulfonic acid, aromaticsulfonic acid and perchloric acid and a solvent; adjusting a pH of thesolution to from about 2.0 to about 3.0. providing a substrate; andelectroplating a selenium-containing film on the substrate using thesolution as a plating bath.
 2. The method of claim 1, further comprisingthe step of: annealing the film at a temperature of from about 100° C.to about 300° C. for a duration of from about 30 minutes to about 60minutes.
 3. The method of claim 1, wherein the solvent is selected fromthe group consisting of: water, glycerol, an ionic liquid, andcombinations thereof.
 4. The method of claim 1, wherein the step ofadjusting the pH of the solution comprises the step of: adding a base tothe solution, after the forming step has been performed, to adjust thepH of the solution to from about 2.0 to about 3.0.
 5. The method ofclaim 4, wherein the base comprises sodium hydroxide.
 6. The method ofclaim 1, wherein the acid is an alkane sulfonic acid, and wherein thealkane sulfonic acid is methanesulfonic acid.
 7. The method of claim 1,wherein the selenium oxide comprises selenium dioxide.
 8. The method ofclaim 1, further comprising the step of: adding one or more additives tothe solution after the pH of the solution has been adjusted.
 9. Themethod of claim 8, wherein the step of adding the one or more additivesto the solution comprises the step of: after the pH of the solution hasbeen adjusted, adding at least one organic additive to the solution at aconcentration of from about 1 part per million to about 10,000 parts permillion.
 10. The method of claim 9, wherein the at least one organicadditive has one or more nitrogen atoms and one or more sulfur atoms.11. The method of claim 10, wherein the at least one organic additive isselected from the group consisting of thiourea and thiazine.
 12. Themethod of claim 9, wherein the at least one organic additive isbenzenesulfonic acid.
 13. The method of claim 8, wherein the step ofadding the one or more additives to the solution comprises the step of:after the pH of the solution has been adjusted, adding at least onemetalloid halide to the solution.
 14. The method of claim 13, whereinthe at least one metalloid halide comprises bismuth chloride.
 15. Themethod of claim 1, wherein the solution comprises at least one metalsalt.
 16. The method of claim 15, wherein the at least one metal salt isselected from the group consisting of: copper sulfate, copper chlorite,copper nitrate, zinc sulfate, zinc chlorite, zinc nitrate, indiumsulfate, indium chlorite, indium nitrate, gallium sulfate, galliumchlorite, gallium nitrate, tin sulfate, tin chlorite, tin nitrate. 17.The method of claim 15, wherein the at least one metal salt comprises acopper salt and an indium salt.
 18. The method of claim 15, wherein theat least one metal salt comprises a copper salt, an indium salt and agallium salt.
 19. The method of claim 15, wherein the at least one metalsalt comprises a copper salt, a zinc salt and a tin salt.
 20. The methodof claim 1, wherein the substrate comprises a molybdenum-coated glasssubstrate.